Electronic properties of amorphous silicon in solar cell operation

Amorphous silicon (a-Si) solar cells, which have efficiencies up to 5.5 percent, are unique in several ways, and their cell characteristics no longer depend on the same parameters generally considered in single crystal cells. This paper discusses the operation of such cells, and relates the major new parameters to the electronic properties of a-Si which are highly dependent on the densities of gap centers in this disordered material. The effect of the photoconductivity in intrinsic a-Si on the series resistance of the cell, the quality of a-Si junctions and their ability to transport current densities generated under AM1 illumination are presented. Space-charge densities as low as 5 × 10¹⁵cm^-3are found in a-Si junctions, which are orders of magnitude lower than previous estimates for glow discharge produced a-Si. The effects of electric field distributions and diffusion lengths of holes on collection efficiencies and cell characteristics are discussed and illustrated. Although significant trapping of photogenerated holes is found in many of the a-Si films, diffusion lengths of few tenths of a micron are also indicated by the results presented.